1. Field of Invention
The present invention relates to a method and circuit for data driving of a display, especially to a method and circuit for data driving of a high resolution and high quality display, such as the poly-silicon TFT or single-crystal silicon TFT display.
2. Related Art
The basic display principle of the liquid crystal display (LCD) is that when a voltage is applied to the rod-shaped crystal molecule in the LCD, the direction of the rod-shaped crystal molecule will be changed, and light will also passes more or less. Therefore, if we can fully fill the space between two parallel glass plates with the liquid crystal and install many vertical and horizontal thin wires on each glass plate, then we can easily control the status of each pixel of the LCD by charging or discharging the corresponding wires, hence control the light. Because the LCD has the advantages of low power consumption, no radiation, fully flat display, and small space occupation, it has gradually replaced the conventional cathode ray tube (CRT) and become increasingly popular.
The thin-film transistor (TFT) is a kind of LCD technologies. For a TFT LCD, each of the pixels of the display is composed of a transistor and capacitors to control the pixel color; furthermore, the pixel information should be kept during the time between two consequent frame refresh of the display. In comparison with the conventional DSTN display, which may not be clearly seen under hard light and/or has only very limited viewing angle, the TFT LCD has greatly improved on these issues and comes into being the mainstream among the LCD technologies.
The data driving circuit used in a conventional TFT LCD usually charges pixels on one of the horizontal gate lines by using the vertical data lines in the manner of 1-to-1 during the scanning time an H-sync signal. During the period, the TFTs on that gate line should be turned on, and all the other TFTs on all other gate lines should be turned off. Since the numbers of pixels on a horizontal gate line are the same as the number of vertical data lines, therefore each vertical data line needs a digital-to-analog converter (DAC) and an operational amplifier as output buffer (OP buffer). For example, for a color display with the resolution of 1024*768, it needs 1024*3 (R, G, B)=3072 DACs and OP buffers. In general, the DAC used in the data driving circuit of a LCD usually utilizes the method of resistor string in conjunction with decoders to determine the voltage. For example, for a 6-bits grayscale driving circuit, it needs 26=64 different voltages. Due to the large chip die area and the complexity in circuit, the above method is rarely used for data driving if the grayscale depth of an image pixel exceeds 8 bits.
FIG. 1 shows a time-to-voltage diagram of a conventional data driving method in the 1-to-1 manner.
For the LCD technologies with better electron mobility, such as poly-silicon TFT and single crystal silicon TFT, the charging time of each pixel is rather short and it is possible to use the 1-to-N data driving method and charge in the manner of time division. During the scanning time H of each H-sync signal, an OP buffer can be used to charge N pixels, that is, the charging time allocated for each pixel is only H/N. For example, for a color display with 1024*768 resolution, it only needs (1024/4)*3 (R, G, B)=768 DACs and OP buffers by using the 1-to-4 data driving method, whereas the charging time of each pixel is only H/4. Although the above method has the advantage of greatly reducing the complexity of circuit, but the charging time of each pixel is short and the charging accuracy is also reduced and thereby affects the grayscale performance of the LCD.
FIG. 2 shows a time-to-voltage diagram of a conventional data driving method in the 1-to-N manner.
As mentioned above, the one of technologic issues on the data driving method of the LCD is to increase the charging time of each pixel while simplifying the structure of 1-to-N data driving circuit so as to improve the grayscale performance.
The primary object of this invention is to increase the charging time of each pixel while simplifying the structure of 1-to-N data driving circuit so as to improve the grayscale performance of a display.
According to the technology disclosed in the present invention, which provides a method and circuit for data driving of a LCD, the data driving processes can be divided into two stages. In the first stage, the part of most significant bits (MSB) of each binary image data is used to do the pre-selection process and then pre-charge data lines simultaneously to around their corresponding voltages, respectively. In the second stage, the part of least significant bits (LSB) of each binary image data is used to do the post-selection process and then charge the data lines in turn to their corresponding voltage, respectively.
In comparison with the conventional 1-to-1 data driving method, the technology disclosed in the present invention utilizes the 1-to-N data driving method so as to simplify the complexity of the data driving circuit. Furthermore, in comparison with the conventional 1-to-N data driving method which utilizes the means of charging in the time division manner, the provided technology according to the present invention comprises not only the means of pre-charging but also pre-chargers so that it has the advantages of increasing charging time of each pixel and greatly improving the grayscale performance.